Developer Wentstorm has released his first version of a new MP3 player called WiiMP3. This is currently limited to playing .mp3 files, has a filebrowser, and some basic controls. The source code is included with the .zip package. See below for the full feature list and download.

Screenshot

Changelog v2.0
* Added support for USB Drives (now you can choose to use SD or USB in the 1st screen)
* In the filebrowser, pressing B while you are in a subdir, makes it to go to the parent directory - if you are in root, it goes back to the drive selection screen
* Added "Now playing FILE" function
* Filesize also displayed in Megabytes
* Volume changing is now possible
* Semplified control's buttons
* While playing a song, pressing B will redirect you to the last visited foder and not in root.
* Added ASCII-Art logo
* Other minor changes